Researchers have long attempted to elucidate the role that viruses play in causing cancer. Worldwide, cancer viruses are estimated to cause 15 to 20 percent of all cancers in humans. Most viral infections however, do not lead to tumor formation as several factors influence the progression from viral infection to cancer development. Some of these factors include the host’s genetic makeup, mutation occurrence, exposure to cancer causing agents, and immune impairment. Viruses typically initiate cancer development by suppressing the host’s immune system, causing inflammation over a long period of time, or by altering host genes.
Cancer Cell Properties
Cancer cells have characteristics that differ from normal cells. They all acquire the ability to grow uncontrollably. This can result from having control of their own growth signals, losing sensitivity to anti-growth signals, and losing the ability to undergo apoptosis or programmed cell death. Cancer cells don’t experience biological aging and maintain their ability to undergo cell division and growth.
Cancer Viruses: Transformation
Transformation occurs when a virus infects and genetically alters a cell. The infected cell is regulated by the viral genes and has the ability to undergo abnormal new growth. Scientists have been able to discern some commonality among viruses that cause tumors. The tumor viruses change cells by integrating their genetic material with the host cell’s DNA. Unlike the integration seen in prophages, this is a permanent insertion in that the genetic material is never removed. The insertion mechanism can differ depending on whether the nucleic acid in the virus is DNA or RNA. In DNA viruses, the genetic material can be directly inserted into the host’s DNA. RNA viruses must first transcribe RNA to DNA and then insert the genetic material into the host cell’s DNA.
Cancer Viruses: Classes
There are two classes of cancer viruses: DNA and RNA viruses. Several viruses have been linked to certain types of cancer in humans. These viruses have varying ways of reproduction and represent several different virus families.
- The Epstein-Barr virus has been linked to Burkitt’s lymphoma. This virus infects B cells of the immune system and epithelial cells.
- The hepatitis B virus has been linked to liver cancer in people with chronic infections.
- Human papilloma viruses have been linked to cervical cancer. They also cause warts and benign papillomas.
- Human herpes virus-8 has been linked to the development of Kaposi sarcoma. Kaposi sarcoma causes patches of abnormal tissue to develop in various area of the body including under the skin, in the lining of the mouth, nose, and throat or in other organs.
- Human T lymphotrophic virus type 1 (HTLV-I), a retrovirus, has been linked to T-cell leukemia.
- The hepatitis C virus has been linked to liver cancer in people with chronic infections.